The present invention relates to a method for controlling a continuously variable transmission installed in a vehicle and more particularly to a method for controlling a rate of changing a RPM ratio between a driving pulley and a driven pulley in a continuously variable transmission.
A major factor influencing the use of a continuously variable transmission in a vehicle is to decrease the amount of fuel consumed by the engine. The continuously variable transmission continuously varies a transmission RPM ratio "e" between a driving pulley and a driven pulley, i.e. (e=a rotating speed of an output shaft/a rotating speed of an input shaft, RPM.sub.out /RPM.sub.in). The transmission efficiency of a continuously variable transmission can be represented as a linearly declining function of the rate of changing the RPM ratio e, represented by e. The rate of changing the RPM ratio (e) is always referred to as a positive quantity, for example, the absolute value of the rate of change is referred to as .vertline.e.vertline. or .vertline.de/dt.vertline..
In general, a requisite horsepower of an engine is typically a function of the amount that a throttle valve is opened. Additionally, a desired engine speed RPM.sub.e ' is determined to have a value corresponding to a minimum amount of fuel consumption when the requisite horsepower for the engine has been determined. The RPM ratio e of the continuously variable transmission is controlled in such a manner that the actual engine rotation speed RPM.sub.e approaches the desired engine rotating speed RPM.sub.e '.
Previous attempts to control the rate of changing the transmission RPM ratio have involved utilizing a fixed rate of changing the transmission ratio, which was completely independent of the amount that throttle valve was opening (.theta.) or closing, i.e. .vertline.d .theta./dt.vertline.. Therefore, if the rate of changing the transmission RPM ratio was fixed at a high value, whenever rapid acceleration of the vehicle was required, the transmission could rapidly respond. However, a large amount of fuel was consumed whenever the vehicle was exposed to steady state driving conditions or when required to slowly accelerate. Conversely, if the rate of changing the transmission RPM ratio was fixed at a small value, only a small amount of fuel was consumed whenever the vehicle was exposed to steady state driving conditions or when slowly accelerating. However, the transmission could not rapidly respond when a rapid acceleration of the vehicle was required.